The requirement for storing energy results especially from the steadily increasing proportion of power plants from the renewable energies sector. It is the aim of energy storage in this case to utilize the power plants with renewable energies in power transmission networks in such a way that renewably generated energy can also be accessed in a time-delayed manner in order to thus save on fossil energy carriers and therefore to save on CO2 emissions.
On account of the time interval which an energy store has to bridge, that is to say the time over which energy is stored in, and extracted from, the energy store, and on account of the capacity which it is required to store, correspondingly high demands are made upon the dimensions of thermal energy stores. On account of the overall size alone, thermal energy stores can therefore become very expensive in their acquisition. If the energy store in addition is of a costly design, or the actual thermal storage medium is expensive in its acquisition or costly in operation, the acquisitioning and operating costs for a thermal energy store can quickly raise doubts over the economical efficiency of energy storage.
So that the production costs for the energy store are affordable, inexpensive storage material is preferred. The heat exchanger should also be dimensioned as most cost-effectively as possible. On account of the often low thermal conductivity of the inexpensive storage materials, the heat exchanger surfaces are often to be of a very large design, however. The large number and long length of the heat exchanger tubes can greatly increase the costs of the heat exchanger in this case, which can no longer be compensated even by an inexpensive storage material.
Up to now, heat exchangers have been designed on the basis of inexpensive materials, chiefly in the form of a direct exchange of the heat transfer medium and the storage material, such as sand or stones, in order to replace large heat exchangers. As heat transfer medium, a working gas, such as air, is used. The working gas in this case can be conducted selectively in a closed or in an open charging circuit or auxiliary circuit.
The fluidized bed technology which is known in principle in engineering has not been used up to now in an order of magnitude which would be necessary for a seasonal storage of renewable surplus energies. A direct heat exchange, moreover, involves a relatively complicated handling of the solid material, which is not economical for a large store.
High capacitive energy stores exist today mainly in the form of pumped-storage power plants and compressed air storage power plants. As a result of the limited extension potential of these stores in many regions, many other storage technologies are in development.
A generic-type energy store can be gathered from WO 2007/137373, for example.